1. Field of the Invention:
The present invention relates to an intake/exhaust valve actuator for actuating an intake or exhaust valve which controls the flow of intake air into or exhaust gases from an engine.
2. Description of the Prior Art:
Conventional actuators for opening and closing intake or exhaust valves usually comprise camshafts and link mechanisms which are incorporated in engines. Therefore, the engines with such conventional intake/exhaust valve actuators are relatively large in size. Since the camshaft is driven by the output shaft of the engine, part of the output power of the engine is consumed by the frictional resistance to the camshafts and the link mechanisms when they are driven. Accordingly, the effective output power to drive road wheels is lowered. As it is difficult to vary the timing with which the intake or exhaust valve is opened and closed, depending on the rotational speed of the engine, the valve opening/closing timing is adjusted at a certain engine rotational speed. As a result, the output power of the engine and its efficiency are reduced when the engine operates at a higher or lower rotational speed.
The above problems can be solved by electromagnets for electromagnetically opening and closing intake or exhaust valves.
Known electromagnetic intake/exhaust valve actuators are disclosed in Japanese Laid-Open Patent Publications Nos. 58(1983)-183805 and 61(1986)-76713, for example. The disclosed valve actuators comprise a movable magnetic pole on the shaft of an intake or exhaust valve, and another magnetic pole fixed to the engine. The valve shaft can be axially moved reciprocally under magnetic forces produced between these magnetic poles, so that the valve can be opened and closed under the control of the valve actuator.
While an intake or exhaust valve is being opened or closed, it can be controlled with a small drive force. However, when the valve is to be opened, it has to be driven against the pressure developed in the engine cylinder, and hence a large drive force is generally required in a direction to open the valve. When the valve closing operation is finished, the valve should be seated on the valve seat without a large shock because the valve would otherwise have a shortened service life. Accordingly, a large drive force is also required in the direction to open the valve in order to decelerate the valve when it is seated.
The intake/exhaust valve actuators disclosed in the above two publications do not have any arrangement for strengthening the valve drive force when the valve starts being opened and stops its closing stroke. Even if a valve opening command is given to the valve at certain timing in response to detection of a crankshaft angle, the valve actuator starts to operate the valve with a certain time lag irrespective of the rotational speed of the engine. Consequently, it is difficult to open and close the valve at such timing that the efficiency of the engine is maximum.
It is necessary to apply a sufficiently large initial drive force to the valve and also to increase the drive force with the rotational speed of the engine. However, an electromagnetic valve actuator for generating electromagnetic forces to produce such drive forces for the control of the opening and closing of an intake or exhaust valve would be large in size.